The present invention relates to an optical fiber inspection apparatus and method, commonly referred to as OTDR, for detecting a spliced point, defective point or breaking point of an optical fiber, or measuring its transmission loss by applying thereinto an optical pulse and measuring the resulting back scattering light.
FIG. 1 shows in block form a conventional optical fiber inspection apparatus. In response to a pulse Ep from a control signal generator 10 an optical pulse generator 11 emits an optical pulse Op, which is applied via an optical directional coupler 12 to one end of an optical fiber under test 13. Back scattering light resulting from the propagation of the optical pulse Op through the optical fiber 13 is incident via the optical directional coupler 12 to an optoelectro transducer 14, by which it is transduced to an electric signal. The electric signal is provided to an A/D converter 15, wherein it is sampled by a sampling clock CK.sub.s of a fixed period .DELTA.t, generated by the control signal generator 10 in synchronization with the pulse Ep, and each sample thus obtained is converted to a digital signal. The digital signal is converted by a logarithmic converter 16 to logarithmic form. The thus converted digital signal F(x) (x indicating the number, 0, 1, . . ., of each sample point) represents the intensity of the back scattering light from the distal or distant end (i.e. the end point) of the optical fiber 13, and for example, as shown on Row A in FIG. 2, the level of the back scattering light lowers as the number x of the sample point becomes larger. At a spliced or defective point of the optical fiber 13 an abrupt attenuation 17 occurs, and at the end or breaking point of the optical fiber 13, a large Fresnel reflection 18 occurs, after which only noise 19 is received.
To lessen the influence of noises, according to the prior art, digital signals of a plurality of successive samples are averaged in a smoothing section 21; for example, a calculation {F(x-1)+F(x)+F(x+1) }/3 is performed, that is, the digital signals of three samples are averaged to obtain date F'(x) on the number of the sample point x. This calculation takes place by steps of three samples while shifting them one by one, that is, a moving average is calculated; thus, an averaged sequence F'(x) shown on Row B in FIG. 2 is obtained. Next, in a difference calculating section 22 a difference between values of the averaged sequence F'(x) at every adjacent sample points, .DELTA.'F(x)=F'(x+1)-F'(x), is calculated to obtain a difference sequence depicted on Row C in FIG. 2. Then the sample point x of that position on the difference sequence .DELTA.F'(x) where its absolute value is greater than a predetermined value is detected in a spliced point detecting section 23. The position on the potical fiber 13 corresponding to the thus detected sample point x is decided to be a spliced point, defective point or breaking point. Letting the sampling interval, the refractive index of the optical fiber 13 and the light velocity in a vacuum be represented by .DELTA.t, n and C, respectively, the distance L along the optical fiber 13 (i.e. the length L of the optical fiber 13) corresponding to the sample number x is expressed by L=.DELTA.t.multidot.x.multidot.C/2n.
As described above, the prior art detects a point of change by calculating the difference in value between adjacent sample points, i.e. through differentiation. Accordingly, if the sampling interval .DELTA.t is shortened to increase the accuracy of measurement of the distance L, the difference in data between adjacent sample points is so small that a large diference value (or differentiated value) .DELTA.F'(x) cannot be obtained at the change point (i.e. the spliced point), and consequently, the detection of the spliced point becomes difficult accordingly. In addition, since the moving average is calculated for smoothing so as to avoid the influence of noise superimposed on the back scattering light, the change of data in the vicinity of the change point 17 becomes dull as shown on Row B in FIG. 2, and hence the difference value (i.e. the differentiated value) .DELTA.F'(x) decreases, making it more difficult to detect the spliced point.
Moreover, the prior art uses a large number of samples and involves the difference calculation after the smoothing operation, and hence requires an appreciably large amount of time for processing. If the sampling interval .DELTA.t is prolonged to make the difference value .DELTA.F'(x) large, then the accuracy of measurement of the distance L is impaired.